1. Field of the Invention
The present invention relates to a piezoelectric resonator, a method for adjusting a frequency of the piezoelectric resonator and a communication apparatus including the piezoelectric resonator and more particularly, a piezoelectric resonator used in an electronic component such as a vibrator, a discriminator or a filter.
2. Description of the Related Art
In a conventional piezoelectric resonator, external electrodes are respectively provided on two major surfaces of a piezoelectric substrate in the form of a rectangular or square plate. The piezoelectric substrate is polarized in the direction of the thickness thereof, and an electric field is applied along the thickness direction of the piezoelectric substrate by inputting a signal between the electrodes to cause the piezoelectric substrate to vibrate in a direction parallel to the major surface of the piezoelectric substrate.
This piezoelectric resonator is an unstiffened type in which the direction of an electric field and the direction of polarization differ from the direction of vibration. The unstiffened type of piezoelectric resonator has an electromechanical coefficient smaller than that of a piezoelectric resonator of a stiffened type in which the direction of an electric field and the direction of polarization are the same as the direction of vibration. The unstiffened type piezoelectric resonator therefore has a comparatively small difference .DELTA.F between a resonant frequency and an antiresonant frequency. This leads to the disadvantage of restricting a bandwidth when the piezoelectric resonator is used as a filter which results in a very small bandwidth when the piezoelectric resonator is used as a filter. Therefore, the degree of freedom in characteristic design of the piezoelectric resonator or an electronic component such as a filter or vibrator using the piezoelectric resonator is low.
The above-described piezoelectric resonator including a piezoelectric substrate in the form of a rectangular or square plate uses first-order resonance of longitudinal vibration. In the above-described piezoelectric resonator, however, strong spurious resonances in high-order modes of odd-number modes, such as the third-order and the fifth-order modes, occur because of the structure of the resonator.
In addition, while the first-order resonance of vibration in the spreading direction is used in the above-described piezoelectric resonator using a piezoelectric substrate in the form of a rectangular or square plate, the possibility of occurrence of strong spurious resonance such as a triple wave in the spreading direction or a spurious resonance in a thickness mode is high.
The applicant of the present invention has previously proposed a piezoelectric resonator of a laminated structure in which spurious resonances are reduced and in which the difference .DELTA.F between a resonant frequency and an antiresonant frequency is large. FIG. 24 illustrates an example of such a piezoelectric resonator having a laminated structure. A piezoelectric resonator 1 shown in FIG. 24 has a laminated structure such that a plurality of piezoelectric layers 3 and a plurality of inner electrodes 4 constituting a base member 2 having a longitudinal direction are alternately laminated and the plurality of piezoelectric layers 3 are polarized along the longitudinal direction of the base member 2. The piezoelectric resonator 1 of this laminated structure is a stiffened type in which the direction of polarization, the direction of electric fields and the direction of vibration are the same. Therefore, the piezoelectric resonator 1 has a larger electromechanical coefficient and a larger difference .DELTA.F between a resonant frequency and an antiresonant frequency in comparison with unstiffened type piezoelectric resonators in which the direction of vibration differs from the direction of polarization and the direction of an electric field. Further, in the piezoelectric resonator 1 having the laminated structure, vibration in a mode such as a width mode or a thickness mode which is different from the fundamental vibration cannot occur easily because of the effect of the stiffened type. In the piezoelectric resonator 1 having the laminated structure, end portions of inner electrodes 4 are exposed at each of the side surfaces of the base member 2. Therefore, the end portions of a first of the two groups of inner electrodes 4 alternating with each other at a first side surface of the base member 2 are covered with an insulating film 5a at one end of the base member 2 in the width direction, and an external electrode 6a is thereafter provided so as to be connected to the other alternate inner electrodes 4. Further, the end portions of the other alternate inner electrodes 4 at the first side surface of the base member 2 are covered with an insulating film 5b at the other end of the base member 2 in the width direction, and an external electrode 6b is thereafter provided so as to be connected to the alternate inner electrodes 4 on which insulating film 5b is provided.
However, if the piezoelectric resonator 1 having the laminated structure shown in FIG. 24 is mass-produced, there is a possibility of failure to obtain a desired resonant frequency or antiresonant frequency due to processing or manufacturing non-uniformity or the like. A method proposed by the applicant and disclosed in Japanese Patent Laid-open Publication No. 197824/1997 can be used as a frequency adjustment method for reducing an obtained frequency to a desired frequency in a situation where the obtained frequency is higher than the desired resonant or antiresonant frequency. However, no method has been proposed as a frequency adjustment method for increasing an obtained frequency to a desired frequency in a situation where the obtained frequency is lower than the desired resonant or antiresonant frequency. With respect to a piezoelectric resonator in which longitudinal vibration is excited, a method of increasing the length of the base member during cutting of surfaces at the ends in the longitudinal direction may be used since a frequency is determined by the length of the base member. In such a case, however, resulting resonator elements vary in length when adjustment is made in order to achieve one desired common frequency. If this process is to be automated, it is necessary to adapt the process in a complicated manner for the desired effect, i.e., jigs made according to the exact length of the piezoelectric resonator are required, and so on. Automation under such conditions is difficult, and a variation in characteristics other than the resonant and antiresonant frequencies also results. In practice, therefore, it is difficult to use such a method.